This invention relates to apparatus for adjusting a frequency characteristic of an electric signal such as an audio signal.
A frequency characteristic adjusting apparatus, which divides an electric signal into a plurality of frequency band signals, and then selectively increases or decreases the amplitude of the band divided signals, is used, for example, as a tone controlling circuit in an audio apparatus.
A prior art frequency characteristic adjusting apparatus, as described in U.S. Pat. No. 2,760,011, includes a band dividing filter, for example, a low-pass filter for filtering the low frequency signal component in an input signal. This prior art apparatus includes a subtraction circuit for subtracting the output signal of the low-pass filter from the input signal thereof so as to obtain the high frequency signal component in the input signal. The thus obtained low and high frequency signal components are amplified respectively, and an adder circuit adds the output signals from the amplifiers. The frequency characteristic of the input signal may be adjusted by increasing or decreasing the gain of the amplifiers. However, such frequency characteristic adjusting apparatus has a following problem. In this apparatus, as shown in FIG. 1, the frequency characteristic of the total gain TG1 is adjusted with amplitude of the low frequency signal component GL1 larger than that of the high frequency signal component G4. On the other hand, the frequency characteristic of the total gain TG2 has the amplitude of the low frequency signal component GS1 and is smaller than that of the high frequency signal component G4. As apparent from these characteristics, the characteristic curve TG1 has a pattern asymmetric to the pattern of the characteristic curve TG2 with respect to the flat frequency characteristic of the total gain obtained while the amplitude of the low frequency signal component is equal to that of the high frequency signal component. Under such condition, the frequency characteristic adjusting apparatus is not a satisfactory tone-controlling circuit.
One method for overcoming this drawback is by varying the cut-off frequency of the filter in response to variation of the amplitude of the low or high frequency signal component. However, the structure of such a circuit is very complex and it is difficult to obtain the correct symmetric frequency characteristic.